Aelfthryth, also known as Elfrida, (d. 929), was the third and last child of Alfred the Great, the Saxon King of England.

She married Baldwin II (d. 918), Count of Flanders. One of their descendents, Matilda of Flanders (d. 1083), would go on to marry William the Conqueror, therefore starting the Anglo-Norman line of Kings of England. Through her descendant, Henry I of England, she is also a direct anscestor of the current monarch of the United Kingdom of Great Britain and Northern Ireland, Elizabeth II.